Strigolactones as a hormonal hub for the acclimation and priming to environmental stress in plants

Abstract Strigolactones are phytohormones with many attributed roles in development, and more recently in responses to environmental stress. We will review evidence of the latter in the frame of the classic distinction among the three main stress acclimation strategies (i.e., avoidance, tolerance and escape), by taking osmotic stress in its several facets as a non‐exclusive case study. The picture we will sketch is that of a hormonal family playing important roles in each of the mechanisms tested so far, and influencing as well the build‐up of environmental memory through priming. Thus, strigolactones appear to be backstage operators rather than frontstage players, setting the tune of acclimation responses by fitting them to the plant individual history of stress experience.


| INTRODUCTION
Strigolactones are a family of plant hormones with a very diversified set of functions. First discovered as root-exuded germination signals for parasitic plants (Cook et al., 1966), they were later assigned their first beneficial role as branching factors for arbuscular mycorrhizal (AM) fungi (Akiyama et al., 2005) and thus, as indirect facilitators of mineral nutrition. In 2008, they were found to be the long-sought branching inhibitor hormone for which the synthesis or perception was compromised in distinct sets of mutants: more axillary growth (max) in Arabidopsis thaliana (arabidopsis); dwarf/high tillering dwarf (d/htd) in Oryza sativa (rice); ramosus (rms) in Pisum sativum (pea); and decreased apical dominance (dad) in Petunia × hybrida (petunia) (reviewed in Al-Babili & Bouwmeester, 2015). More functions have been added in the following years, namely for root development and plasticity (reviewed by Mitra et al., 2021). Shortly thereafter, it became clear that strigolactones are involved in the response to abiotic stresses, first of all, nutrient deficiency and more recently, osmotic stress among others (Andreo-Jimenez et al., 2015;Cardinale et al., 2018).
Among the many possible environmental constraints to plant growth, drought is one of the main ones (Bodner et al., 2015). Indeed, low water availability is a challenging condition for all living cells; understanding to what extent organisms can 'learn' to cope with drought stress episodes remains a fascinating question in basic biology that plants are optimally suited to answer. To loosely categorize, there are three types of strategies traditionally identified, which plants can adopt to increase their chance of survival in the face of low water availability: avoidance, tolerance and escape (Levitt, 1980). Different plants may have evolved priority for individual strategies, but still have the whole set of response strategies available, which can be co-opted to a range of abiotic stresses.
Broadly speaking thus, these categories can depict responses to all environmental stresses and will be used here as a more general paradigm for acclimation strategies in plants.
The first strategy is avoidance, which is achieved by optimizing water uptake by roots (in the so-called 'water spenders'), or by increasing water use efficiency and conservation by closing stomata and thickening the leaf epidermis and cuticle (in 'water savers'). The second is tolerance, which is conspicuously acquired via the mitigation of stress impact by structural and biochemical mechanisms. The third is escape, which is mainly achieved by altering the pace of the developmental programme as to shorten the life cycle, and flower shortly after drought recovery. It must be emphasized that the boundaries between these schematic paths may be blurred, in that plants may adopt a combination of strategies depending on the species, but also on the intensity and the speed of stress onset (severe vs. mild, sudden vs. gradual), which affects the ability of the plant to acclimate to the incoming stress (e.g., Morabito et al., 2022;Ruehr et al., 2019). Also, the previous stress experience of the plantbe it of the same or different nature-steers the plant response towards a strategy that it would not necessarily favour if stress naïve.
Building an 'environmental memory' can thus help the plant to better acclimate to its specific, fluctuating environment. Indeed, in the most common situation, the first period of sub-lethal stress can prepare (prime) the plant for a subsequent (challenging) stress either similar or dissimilar in nature, thus triggering a form of plant memory (Bäurle, 2018;Kinoshita & Seki, 2014;H. Liu, Able, et al., 2022).
In the frame of such classic interpretation of plant responses to stress, this review will focus on the most recently reported evidence of strigolactone contribution to stress avoidance, tolerance and escape. The picture we will try to sketch is that of a hormonal family playing important roles in most aspects of environmental stress acclimation mechanisms tested so far, influencing as well the build-up of environmental memory through priming.

| LATEST DEVELOPMENTS IN THE SYNTHESIS, PERCEPTION AND TRANSPORT OF STRIGOLACTONES
A detailed review of current knowledge of strigolactone synthesis up to 2021 can be found in (Yoneyama & Brewer, 2021), while perception, signalling and transport are excellently covered by (Mashiguchi et al., 2021), to which the reader is directed. For the purpose of this review, the main molecular players will be outlined concisely and a few more recent findings, not reported in the above reviews, will be highlighted.

| Biosynthesis
Strigolactones are a large family of compounds currently comprising over 30 members and growing. They can be subdivided into canonical strigolactones (containing an ABC-ring connected via an enol-ether bridge to a methylbutenolide D-ring) and non-canonical ones, in which the A, B or C rings are missing. A further distinction is between strigol-or orobanchol-type strigolactones, depending on the orientation of the C ring (Yoneyama & Brewer, 2021); with few exceptions, individual plant species produce a blend of either type. Strigolactone synthesis starts in plastids, by the sequential action of three conserved enzymes on all-trans-β-carotene to produce the key intermediate carlactone, a compound sharing with strigolactones the number of C atoms and the presence of a butenolide ring. Such core enzymes are, in biosynthetic order, the carotenoid isomerase DWARF27 (D27) and the carotenoid cleavage dioxygenases (CCD) 7 (MAX3/D17/HTD1/RMS5/DAD3) and CCD8 (MAX4, D10, RMS1 and DAD1) (Yoneyama & Brewer, 2021).
Additionally, as shown in rice, the core biosynthetic module (D27, D10, D1) can also act on a hydroxylated derivative of all-trans-β carotene (zeaxanthin) leading to hydroxylated carlactone, which may in turn act as the precursor of as-yet unidentified strigolactones (Baz et al., 2021).
Beyond arabidopsis, the pathway downstream of carlactone to generate different structures of canonical strigolactones may entail, for example, several paralogues of the CYP711A MAX1 cooperating sequentially, as in rice (Yoneyama et al., 2018;Y. Zhang et al., 2014).
It may also recruit, downstream of MAX1 orthologues, other CYP450 enzymes such as CYP722C isoforms in cotton (Gossypium arboretum), tomato (Solanum lycopersicum) and cowpea (Vigna unguiculata) (Wakabayashi et al., 2019(Wakabayashi et al., , 2020, or the CYP728B35 LOW GERMINATION STIMULANT1 (LGS1) in sorghum (Sorghum bicolour) (Gobena et al., 2017;Wu & Li, 2021). The CYP712G1 enzyme of tomato was very recently shown to oxidize orobanchol to didehydroorobanchol isomers, one of which can be further converted to solanacol (Y. Wang, Durairaj, et al., 2022). Finally, one last important piece of the biosynthetic jigsaw has been recently identified in the form of the first catabolic enzyme for strigolactones, named CARBOXYLESTERASE15 (AtCXE15) in arabidopsis. This enzyme, and its orthologues in other tested species, can degrade both canonical and non-canonical strigolactones; AtCXE15 and, possibly, paralogues in this large family such as AtCXE20 or others are important in finetuning the effects of strigolactones on shoot branching  and stress (Roesler et al., 2021).

| Perception and signalling
Also our understanding of strigolactones perception and signalling has progressed fast. With respect to the core signalling pathway, elucidated several years ago (Mashiguchi et al., 2021), the main advancements have come from new hypotheses on the origins of strigolactone signalling in land plants, and from a better molecular understanding of the perception and early transduction steps.
Strigolactone perception starts by physical binding to the receptor D14 (DAD2/RMS3), an α/β-fold hydrolase protein which retains enzymatic activity but with extremely low turnover. This implies that the hormone is rather slowly hydrolysed to a tricyclic ABC and a D-ring moiety (Hamiaux et al., 2012). Initially, it was postulated that ligand hydrolysis is necessary for strigolactone signalling (De Saint Germain et al., 2016;R. Yao et al., 2016); however, to accommodate some inconsistencies (Carlsson et al., 2018;Seto et al., 2019), hydrolysis-independent signalling mechanisms have been proposed more recently.
It must be emphasized that there is not yet complete consensus on the finer details, especially regarding the sequential changes in molecular conformations and the role of the covalent modifications of D14 needed to achieve optimal sensitivity (Bürger & Chory, 2020;Mashiguchi et al., 2021). Nonetheless, according to one of the most recent interpretations, what triggers initial signalling is not the covalent binding of a hydrolytic by-product of strigolactones in the active site of D14, but rather the destabilization of the latter upon physical interaction with the intact ligand. Once the D14-strigolactone complex is formed, D14 destabilization makes it competent to bind the F-box protein MAX2 (D3/RMS4) (Zhao et al., 2015), a key transducer in the signalling cascade.
MAX2-D14-strigolactone can then recruit a member of the D53/ SMXLs (SUPPRESSOR OF MAX2 1-LIKE) family, and subsequently mediate its polyubiquitination (Figure 1 right). Additional intramolecular changes within D14 upon strigolactone hydrolysis, and in MAX2, allow the release of ubiquitinated D53/SMXLs to the proteasome, their degradation and thus, transcriptional derepression of strigolactone-responsive genes (Tal et al., 2022). It has been shown that among the members of the SMXL family, SMXL6/7/8 are responsible for the repression of most strigolactone signalling under regular conditions in arabidopsis (L.   (Figure 1 right). It is notable that SMXL6 also acts as a transcription factor targeting its own gene, among many others (Tang & Chu, 2020;. Other MAX2 interactors have been identified: one of them is bri1-EMS-suppressor 1 (BES1), which was initially characterized as a positive regulator in the brassinosteroid signalling pathway and later proposed as a regulator of strigolactone-dependent shoot branching (Y. Wang et al., 2013).
Another is, in rice, the DELLA suppressor of gibberellin signalling, SLENDER RICE1 (SLR1) (Nakamura et al., 2013), and although the relevance of the latter interactions for shoot branching inhibition seems dubious (Bennett et al., 2016) degradation, enabling feedback regulation of the signalling cascade (Chevalier et al., 2014;Tal et al., 2022). Compound to ligand hydrolysis and persistent receptor occupation by the ligand, this step may gradually desensitize those tissues in which strigolactone signalling becomes intense (Chevalier et al., 2014;Seto et al., 2019).
D14 receptors include members that have undergone wide differentiation in plants and beyond. A recent study found a D14-like protein in the phytopathogen Cryphonectria parasitica that interacts with strigolactones, hydrolyses them and is needed for their perception, even though the ecological significance of strigolactones for this organism is unresolved (Fiorilli et al., 2022). Also the perception of strigolactones as germination stimulants in seeds of parasitic plants relies on an expanded family of D14-like receptors, called HYPOSENSITIVE TO LIGHT (HTL). HTL genes turned out to be present in all seed plants but with a possible different role than D14 proper. The first HTL gene discovered in arabidopsis was called KARRIKIN INSENSITIVE2 (KAI2) because its mutant, while not affected in strigolactone perception, is insensitive to compounds named karrikins. These are abiotic in origin, as they are smoke components produced by combustion of plant tissues; they act as germination stimulants for fire-succession species and structurally share a butenolide ring with strigolactones (Waters et al., 2012). However, while strigolactones have a butenolide ring with a methyl group that is essential for bioactivity, the corresponding methyl group of karrikins is nonessential for perception by KAI2, which indeed seems to prefer desmethyl butenolides (J. Yao et al., 2021). Perception of karrikins seems to have no significant biological role in arabidopsis, so KAI2 is thought to perceive primarily an endogenous and yet unknown molecule tentatively named KAI2 ligand; its structure is expected to share structural resemblance to karrikins, at least in its essential features (Conn & Nelson, 2016) (Figure 1 right).
The analogies and intersections between the two sibling pathways, either triggered by strigolactones or KAI2 ligand/karrikins, are not limited to the ligand and receptor (Q. Wang, Smith, et al., 2022).
MAX2 is used by both (Nelson et al., 2011), and this realization has forced a revaluation of all initial attributions to strigolactones of max2 phenotypes. The fact that most of the early works made use of a racemic mixture of the strigolactone analogue GR24 (rac-GR24) to describe the effects of exogenous strigolactones further obfuscated the picture, since it contains two stereoisomers, GR24 5DS and GR24 ent-5DS (Figure 1 right). The latter, albeit carrying an unnatural stereoconfiguration, stimulates both D14 and KAI2 to a certain extent, at least in arabidopsis . The former instead, which has a 5-deoxy-strigol configuration, seems to be more specific for D14-even though under certain conditions, it may be perceived weakly also by KAI2 (Villaecija Aguilar et al., 2019). More recently, it was found that one of the two other possible GR24 stereoisomers, GR24 4DO (which reflects the deoxy-orobanchol configuration and is normally not present in commercial rac-GR24 mixtures) is less potent than GR24 5DS but more specific to D14 stimulation in arabidopsis (L.   (Figure 1 right). This highlights how care should be exerted in attributing pharmacological findings to either pathway. For this same reason, recent works trying to untangle this knot have made use of pathway-specific mutants coupled to more specific ligands, for example, karrikins vs GR24 5DS , karrikins vs GR24 4DO (L. , or GR24 ent-5DS vs GR24 4DO (L. Wang, Xu, et al., 2020) to stimulate KAI2 versus D14, respectively. Additionally, proteins that are clear KAI2 paralogues from their primary sequences can exhibit divergent ligand stereoselectivity and even have affinity for strigolactone-type molecules, as in the case of KAI2A and KAI2B in pea (Guercio et al., 2022), making in silico-only functional predictions less reliable than in other cases.
The evolutionary origins of strigolactone and KAI2 ligand/ karrikin signalling components are being investigated actively, in an effort to pinpoint, among the many effects of this molecular family, which is ancestral and which derived. The presence of the core biosynthetic pathway (D27, CCD7, CCD8) in green algae, before the evolutionary appearance of obvious perception and signalling components, rather suggests an exogenous role for ancestral strigolactones. This hypothesis is strongly supported by the recent identification of the novel strigolactone molecule bryosymbiol in the bryophyte Marchantia paleacea, which lacks the ability to perceive it, but uses it to attract mycorrhizal partners in the rhizosphere (Kodama et al., 2022). With respect to the strigolactone versus KAI2 ligand/ karrikin perception pathways, initial hypotheses had placed KAI2 as ancestral to D14 (Waters et al., 2012). The complete pathway seems to have been achieved in bryophytes after gradual acquisition of KAI2, MAX2 and SMXLs (Delaux et al., 2012;Q. Wang, Smith, et al., 2022). In turn, phylogenetic analyses suggest that plant KAI2 was

| Transport
Regarding strigolactone transport, we refer the reader to recent reviews such as Mashiguchi et al. (2021), and only wish to mention here the recent identification and characterization of orthologues of the petunia protein PDR1 (PLEIOTROPIC DRUG RESISTANCE 1), the first membrane transporter shown to be involved both in strigolactone exudation in the rhizosphere, and in cell-to-cell strigolactone movement (Kretzschmar et al., 2012;Sasse et al., 2015). PDR1 is a member of the ancient and ubiquitous ATP-binding cassette (ABC) protein family and belongs to the ABCG subfamily, which is the most numerous in plants.
The relatively high similarity among the many family members has slowed down the transfer of results from Petunia spp to other species; the recent functional characterization of PDR1 orthologues in Medicago truncatula and tomato is a welcome advancement (Banasiak et al., 2020;Bari et al., 2021). It is important to notice that at least in arabidopsis, carlactone, carlactonoic acid and methyl carlactonoic acid are actively translocated from root to shoot via PDR1, and may be themselves the main bioactive strigolactones in this species (Mashiguchi et al., 2022). It is unclear, for plant species that produce also canonical strigolactones, whether only mature strigolactones are transported systemically via PDR1, and/or their precursors. The hypothesis that there might be two different routes of transport in the shoot has also been advanced, one to distribute locally synthetized strigolactones to adjacent tissues, and one to transport them across a long distance ( and not touched upon here). Since 2014, the contribution of strigolactones to osmotic stress responses has been rather extensively documented as well. We will try to highlight their role in both avoidance and tolerance-based acclimation strategies to abiotic stress, with non-exclusive emphasis on osmotic stress.
3.1 | The model for osmotic stress avoidance: Organ-specific strigolactone dynamics Initial reports pointed out how strigolactone-insensitive or depleted lines of arabidopsis (Ha et al., 2014), Lotus japonicus (lotus) (J. Liu et al., 2015), tomato (Visentin et al., 2016) and barley (Marzec et al., 2020) are hypersensitive to osmotic stress and wilt more easily than their wild-type counterparts when water becomes scarce. Note that in plants, osmotic stress is usually triggered by reduced irrigation and/or high salinity, but also-namely, under laboratory conditionsby the presence of osmotically active compounds such as mannitol or polyethylenglycol (PEG). The above hyper-sensitivity is generally linked to lower relative water content, impaired photosynthesis, and higher stomatal conductance, coupled to sometimes altered stomatal density, larger stomata and/or slower stomata closure in response to drought. This latter feature in turn has been linked to defects in abscisic acid (ABA) sensitivity and, less consistently, ABA synthesis Higher stomatal conductance for similar leaf water potential is more obvious under irrigated and mild stress conditions, while under severe stress, hydraulic signals seem to prevail and no differences are often seen in stomatal conductance of strigolactone-related mutants and wild-type (Visentin et al., 2016). In accordance with the above features, transcriptomics on d14 mutants point to a wide dysregulation of stress-and ABA-responsive genes in arabidopsis plants undergoing drought, even though an ABA-independent signature is detectable (W. Li, Nguyen, Chu, et al., 2020). Rice seems to be the only exception in this picture, since mutants lacking proteins downstream of D27 in the biosynthetic pathway are reported as more drought tolerant than the wild-type, due to the-so far uniquenegative correlation between strigolactone and ABA contents in this species (Haider et al., 2018) that was confirmed later (X. Liu et al., 2020). What the significance of such specific difference may be in rice ecology awaits investigation.
Consistent with the hypersensitive phenotype of the strigolactone mutants, drought induces the transcription of strigolactone biosynthetic genes in the leaves of all tested species, such as arabidopsis and tomato (Ha et al., 2014;Visentin et al., 2016;Visentin et al., 2020)-although it has so far proven impossible to detect the metabolites in aerial tissues. This is likely due to a technical sensitivity issue: even in the presence of drought-induced activation, the transcripts of the biosynthetic genes are about 100-fold less concentrated in the leaves than in unstressed root tissues (Visentin et al., 2016). Furthermore, biosynthesis may be localized to only certain cell types, leading to dilution and/or the production of strigolactone variants of undescribed structure that will go undetected. The strigolactone scientific community still misses the broad availability of a genetically encoded strigolactone sensor or reporter, allowing the direct visualization of sites and intensities of strigolactone action at the cell level in planta. Some attempts have been made in this direction (Samodelov et al., 2016;Sanchez et al., 2018;Song et al., 2022) cite only the most obvious (see above). Especially Strigo-D2 seems a promising tool nonetheless, because cell-level definition in live tissues appears good and calibration reliable (Song et al., 2022).
However, it still needs to be tested under stress conditions.
This decrease is needed for local ABA rise in lotus (J. Liu et al., 2015), and sufficient for the activation of the strigolactone biosynthetic pathway in the leaves of tomato (Visentin et al., 2016). Thus, a model has been put forward in tomato whereby the drop in root-synthesized strigolactones is sensed by the shoot, where it triggers a local increase in strigolactone synthesis, for improved drought acclimation Visentin et al., 2016). It appears also that this organ-specific biosynthetic pattern of strigolactones is unique for osmotic stress, as other cues that may carry an osmotic component, such as heat stress, induce an increase of strigolactones in the roots (Chi et al., 2021). It is notable, and calls for more investigations, that the issue has not yet been addressed in arabidopsis.
How systemic sensing happens is also interesting and still unresolved. It may happen indirectly, with the strigolactone decrease in the roots triggering an as yet undefined shootward signalling pathway of which the mediator(s) are unrelated to strigolactones.
However, given the demonstrated upward mobility of strigolactone themselves in the xylem parenchyma, and their ability to repress their own biosynthetic pathway, it may also occur directly, with the loss of root-derived strigolactones de-repressing synthesis in the shoot under drought . This is a key point that needs further investigation.
Additionally, with all the caveats described above about the use of rac-GR24 versus its more specific enantiomers (Figure 1 right), shortterm stomatal closure in the absence of stress has been observed in a number of species upon GR24 treatment, among which arabidopsis , faba bean (Vicia faba L.) (Y. Zhang et al., 2018) and tomato (Visentin et al., 2016). This was shown to be ABA-independent and dependent on hydrogen peroxide (H 2 O 2 ) and nitric oxide production in arabidopsis . Given the effect of strigolactones on hydrogen sulphide synthesis (Huang et al., 2021)  Finally, it must be also noted that mycorrhization and increased root:shoot biomass ratio are also considered ways to avoid not only The influence of strigolactones (SLs) on the main water-spending and water-saving mechanisms in drought avoidance. Strigolactones support water savings by promoting ABA-dependent and independent stomatal closure, and delayed stomata re-opening during drought recovery (the 'after-effect' of drought, a feature of drought stress memory). They also contribute to drought-triggered leaf epidermis and cuticle changes, for which the main driver is however the KAI2 ligand/karrikin pathway in arabidopsis. Finally, they can ensure the sustainability of a water-spending approach by favouring arbuscular mycorrhizal (AM) symbiosis and biomass allocation to the roots, thus allowing better soil exploration and water capture. [Color figure can be viewed at wileyonlinelibrary.com] nutrient-but also water-related stress, namely in water spenders.
Thus, the promotion of mycorrhization (not treated here, see Lanfranco et al., 2018) and of root biomass allocation (see below) by strigolactones can be seen as yet another aspect of their positive influence on stress avoidance. In tomato, the conserved microRNA miR156 is dependent on strigolactones for drought stress induction, and miR156 overexpression intensifies the after-effect of drought also by increasing ABA sensitivity at the guard cell level, but not total leaf ABA content (Visentin et al., 2020). The more-than-additive synergy between strigolactone treatment and drought in miR156 induction during recovery is remarkable (Visentin et al., 2020), and points to a possible effect of GR24 5DS in priming for drought responses at the level of mature miR156 production. Whether this priming is also detectable in repeated drought stresses either at the miR156 or stomatal level, is still an open question. It is worth noting, however, that many arabidopsis genes, of which the expression profile is strigolactonedependent in the recovery phase after drought, were also defined as 'memory genes' for repeated dehydration stress in previous works (Ding et al., 2013;Hopper et al., 2014;Korwin Krukowski et al., 2022;Virlouvet & Fromm, 2015). Also, miR156 is important for the memory of repeated heat stress in arabidopsis (Stief et al., 2014)
A few studies make use of genotypes altered in the synthesis or perception of strigolactones to draw conclusions on their role in stress mitigation. The overexpression in arabidopsis of the MAX2 homolog SsMAX2 from the popcorn tree Sapium sebiferum confers resistance to osmotic stress, and increases proline and soluble sugar content as well as CAT, SOD and POD activity when compared to the wild-type. As a likely consequence, H 2 O 2 and MDA concentrations are lower in the SsMAX2-overexpressing lines, while the max2 mutant has the opposite molecular and physiological phenotype (Q. Wang et al., 2019). Of course, these results could be ascribed to the D14-dependent pathway but also, as not emphasized enough by the authors, to the KAI2dependent one (see dedicated paragraph below). Nonetheless, the salinity-sensitive phenotype of strigolactone-only mutants, such as max3 and max4, is documented (Ha et al., 2014), so the pathway contribution in the phenotype of SsMAX2-overexpressing arabidopsis remains likely. Also, the role of MAX2, MAX3 and MAX4 in drought resistance was originally assessed by the use of mutants (Ha et al., 2014). Consistently, the triple mutant smxl6/7/8, more than the single and double mutants of these repressors of strigolactone action, has a better survival rate after drought than the wild type and, of course, than the max2 mutant (T. Yang et al., 2020). In this picture, the only exception seems to be rice, for which-as mentioned above-all strigolactone-related mutants but d27 are more drought-tolerant than the wild type (Haider et al., 2018).
4.2 | The acclimation dilemma: To avoid stress, to tolerate it, or both?
In winter wheat subjected to drought stress, treatment with rac-GR24 and/or salicylic acid increases membrane stability and the activity of POD, CAT, APX, SOD-especially when combined. For some of the tested parameters, the hormonal treatments are beneficial also on irrigated plants, which display lower MDA content and electrolyte leakage, and higher APX activity with respect to the untreated, unstressed plants (Sedaghat et al., 2017). Notably, a strigolactone-salicylic acid crosstalk has been recently suggested in the frame of biotic stress responses as well (Kusajima et al., 2022). Sedaghat et al. (2017) have also ascertained that winter wheat performances under drought are improved to slightly different extents depending on whether GR24 is applied on the roots or leaves, and also that root application is especially effective at increasing root biomass. This in itself can justify more efficient water capture by treated plants, which can afford to keep stomata more open and sustain photosynthesis better than untreated, stressed plants. In the case of GR24-treated winter wheat under drought, we thus observe both better tolerance via increased antioxidant defences (SOD, CAT, POD, APX activity), and the parallel adoption of a water-spending strategy (in itself, a feature of drought avoidance) (Sedaghat et al., 2021).
The increase in root biomass following GR24 treatment was also observed in other plants su tomato and switchgrass (Panicum virgatum L.) (Tai et al., 2017;Santoro et al., 2020); consistently, strigolactone-depleted genotypes tend to have lower root/shoot biomass ratios, although with exceptions (Rasmussen et al., 2013). In a further example, cadmium stress is both better tolerated in GR24treated plants, and in part avoided, since it appears that treatment helps the plant exclude the toxic metal, reducing its uptake (Qiu et al., 2021;Tai et al., 2017). A similar situation is reported for arsenate (Mostofa, Ha, et al., 2021;. Finally, rac-GR24 triggers a more efficient water-saving phenotype (and thus, drought avoidance) in leaves of grapevine (Vitis vinifera L.) under PEG-induced osmotic stress, by allowing to maintain net carbon assimilation rates in the presence of lower stomatal conductance.
GR24 treatment is also associated to lower electrolyte leakage and increased water and chlorophyll content compared to the nontreated, stressed plants, which may be seen as both a feature of increased stress tolerance, and of avoidance. In this experimental setup, though, the enzymatic and non-enzymatic antioxidant system was less activated and yet, the H 2 O 2 and MDA content were lower in GR24-treated plants. This suggests that stress was rather avoided than better tolerated; however, the work lacks appropriate controls on GR24-treated, unstressed plants (Min et al., 2019).
To conclude, it is important to keep in mind that depending on the plant species, the stress conditions and the mode/timing/dose of administration, GR24 may push the plant to mostly avoid rather than mitigate osmotic stress consequences, or vice versa; or yet, to do both. It is possible, for example, that if stress occurs fast and is severe, tolerance will not have time to become effective; and a stress avoidance strategy may rather be deployed. It is known indeed that different stress application protocols can lead to different outcomes and physiological responses, for example in woody plants (Morabito et al., 2022;Ruehr et al., 2019). In tomato, GR24 5DS delivered before abrupt dehydration clearly instigates a water-saving strategy (Visentin et al., 2020), while under different conditions, rac-GR24 induces tolerance to heat and cold stress (Chi et al., 2021 While the general processes affected by the two pathways largely overlap in relation to drought, there are some specificities at a more granular level. For example, KAI2 seems to have a stronger role in the maintenance of cell membrane integrity, leaf cuticle structure and ABA-induced leaf senescence than D14, but a weaker role in drought-induced anthocyanin biosynthesis and effect on brassinosteroid and cytokinin signalling. Instead, both pathways affect photosynthesis and the metabolism of glucosinolates and trehalose, and an additive effect is suspected in regulating cell membrane integrity and leaf cuticle development (W. Li, Gupta, et al., 2020;W. Li, Nguyen, Chu, et al., 2020). It is also interesting to notice that even within the same biological process, different genes may depend from either pathway for proper regulation under drought. For example, specific gene subsets in the ABA-responsive category are misregulated in the kai2 or d14 mutants under drought stress (W. Li, Gupta, et al., 2020;W. Li, Nguyen, Chu, et al., 2020). The only partially redundant action of D14 and KAI2, and the fact that both signalling pathways converge at MAX2, justifies the fact that the hypersensitivity to osmotic stress of max2 or d14kai2 plants is more severe than either kai2 or d14 individual mutants (Bu et al., 2014;W. Li, Nguyen, Chu, et al., 2020).

| STRIGOLACTONES AS REGULATORS OF STRESS ESCAPE: A POSSIBILITY WORTH TESTING
A missing part of the picture is whether strigolactones may also regulate stress escape by affecting the timing of flowering. While direct evidence is sorely needed, several clues suggest that they might, especially in certain plant species. First, strigolactones affect sensitivity to ABA, a hormone that has been proven important for drought escape (Riboni et al., 2016). Second, strigolactone-related mutants in several (but not all) plant species have reproductive defects. For example, knocking down the biosynthetic gene CCD7 makes lotus produce fewer flowers, fruits and seeds (J. Liu et al., 2013). Among solanaceous plants, the most severely affected potato (Solanum tuberosum L.) lines silenced for CCD8 do not flower at all (Pasare et al., 2013); and in petunia, delayed flowering time and smaller flowers have been reported for analogous lines (Snowden et al., 2005). In tomato, CCD8 silencing causes fewer and smaller flowers and fruits ; and in pea, max2 plants have delayed flowering (Rasmussen et al., 2015). So far, little effort has been put into exploring the molecular underpinnings of this phenotype; and even less for a possible intersection between strigolactones, stress and flowering. However, several floweringrelated genes are misregulated in the comparison between wild-types and strigolactone-related mutants, both in the absence and in the presence of drought (Ha et al., 2014;Korwin Krukowski et al., 2022).
Thus, the next obvious experiments should test the possibility that strigolactones may indeed regulate flowering time in plants experiencing osmotic stress, both in species that do show strigolactonerelated flowering phenotypes under normal conditions-such as solanaceous plants and some leguminous-and species that do not, such as arabidopsis and rice.

| CONCLUSIONS
The number of open questions regarding the functions of strigolactones in stress acclimation are undoubtedly still more numerous than the ones we have answered so far. Nonetheless, it appears clear that strigolactones do play a pivotal role in potentially all main codified aspects of the process (Figure 4). It may be argued that this stems from their modulation of ABA sensitivity/synthesis, a hormone that has long occupied a centre stage position in plant stress biology for its pervasive effects on avoidance, tolerance and escape.
However, the sheer fact that strigolactone action also displays ABA-independent features shows their peculiarities. Additionally, their effects in stress memory and priming, although certainly exerted in part via ABA, highlight how strigolactones may be rather setting the level of alert towards fluctuating environmental conditions, than act as direct effectors of stress acclimation responses. As such, their function would be subtler than the one of ABA, but as important for plant resilience in the long term.